The present invention relates to downhole tools for use in the oil and gas industry and in particular although not exclusively to a disengagable downhole tool which allows a tool to be disengaged from a work string in a well bore and later retrieved on the work string when the work string is removed from the well bore.
In drilling or completing a well, each stage requires a work string to be made-up which includes any tools required within the well bore. Typically once made-up the work string is inserted in to the well until the tool reaches the desired location, the job is undertaken and then the work string including the tool is returned to the surface. As a number of jobs are required in a well in order to drill and complete the well, this can require a great number of trips into the well by a work string. Each time the string is retrieved, made up and reinserted time is lost which increases the cost involved in drilling and completing the well.
It has long been known to combine tools on a single work string. However, some tools such as the drill bit can only be located at a single position on the work string e.g. at the base.
An example where a single tool is required to be placed on a work string is in the milling of a polished bore receptacle. A polished bore receptacle is typically positioned at the top of a production liner in conjunction with the liner hanger disposed in a well bore. The polished bore receptacle typically has a long polished bore, which slideably and sealingly receives a sealing assembly on the end of a tubing string. Due to its function of requiring a surface against which a seal can be made, the polished bore receptacle which is inserted into the well is generally milled and dressed to provide an ideal surface finish. Such milling and dressing of the receptacle and in particular the top portion of the liner requires a single trip into the well with a suitable milling assembly.
Where multiple tools can be mounted on a single work string, difficulties can arise in the need to co-ordinate the activities of each of the tools independently from each other, particularly, if one tool is required to work before or after the operation of another tool.
Further difficulty arises when a tool, in order to operate, must come into contact with a portion of the well bore lining or casing. In these circumstances, the tool must be capable of being retracted or moved away from the well bore lining or casing so that the work string can be repositioned without the tool making unwanted contact to other parts of the well bore.
An example of a retractable tool is that disclosed in U.S. Pat. No. 2,346,629. This tool is mounted on a work string and operates by the application of fluid through the work string. Fluid pressure causes cleaning members in the form of brushes or scrapers to be expanded radially outwards to contact the walls of the well casing or liner. The work string can be rotated so that the brushes or scrapers clean the walls of the casing or liner. When cleaning is complete a change in fluid pressure causes the cleaning members to be retracted back into the work string so that the work string may be lowered further into the well or be retrieved from the well without the cleaning members making any further contact with the casing or liner. A disadvantage of such systems is that they require changes in fluid pressure from the surface and due to the mechanical components used to assist in the expansion and retraction of the cleaning members they can be prone to failure in hostile environments.
It is an object of at least one embodiment of the present invention to provide a tool located on a work string, which when it has completed its function in a well bore can be disengaged from the work string such that the work string be run further into the well bore and when retrieved ‘pick-up’ the tool and remove it from the well bore.
It is a further object of at least one embodiment of the present invention to provide a tool for insertion in a work string including at least one further tool, which when the string is inserted into the well bore can mill a polished bore receptacle, remain at the polished bore receptacle while the one or more further tools perform their function(s) below the polished bore receptacle and is retrieved when the work string is retrieved from the well.
It is a yet further object of at least one embodiment of the present invention to provide a tool for insertion in a work string which includes a safety feature such that a portion of the tool will disengage only when the tool has reached a desired location in the well bore.